Process for the recovery of metals

ABSTRACT

Metal values (especially uranium values) are extracted from aqueous solutions of metal oxyions in the absence of halogen ion using an imidazole of defined formula. Especially preferred extractants are 1-alkyl imidazoles and benzimidazoles having from 7 to 25 carbon atoms in the alkyl group.

This is a continuation of application Ser. No. 08/021,669, filed on Feb.24, 1993, now abandoned, which is a continuation of Ser. No. 07/845,822filed Mar. 6, 1992, now U.S. Pat. No. 5,213,777, which is a continuationof Ser. No. 07/585,277 filed Sep. 14, 1990, now abandoned, which is acontinuation of Ser. No. 07/443,052 filed Nov. 28, 1989, now abandoned,which is a continuation of Ser. No. 07/155,802 filed Feb. 16, 1988, nowabandoned.

This invention relates to a process for the recovery of metals, and inparticular to a process for the recovery of metals capable of formingoxyions in aqueous solution.

The use of solvent extraction techniques for the recovery of metals hasbeen practised for a number of years, and was first applied commerciallyon a large scale to the extraction of uranium from its ores. Typicallythe uranium ore is milled to a suitably small particle size and isleached with dilute sulphuric acid to form an aqueous solutioncontaining the uranyl ion, UO₂ ²⁺, which may exist as complex salts withone or more sulphate ions such as UO₂ SO₄)₂ ²⁻ or UO₂ (SO₄)₃ ⁴⁻. It isto this solution that the solvent extraction technique is applied.Commercially, uranium is one of the most significant of the metalscapable of forming oxyions in aqueous solution.

A large number of compounds have been proposed as extractants for metalssuch as uranium, but few have proved to be commercially successful. Themost widely used extractant for uranium at the present time istri-n-octylamine, which typically is used in a solvent extraction planthaving up to five extraction stages and up to four strip stages. Eachstage requires a separate mixer-settler, and in consequence a large andexpensive plant is required.

The present invention seeks to provide an improved extractant for therecovery of metals capable of forming oxyions in solution, and inparticular a reagent capable of operating with a reduced number ofextraction and/or strip stages.

In United Kingdom Patent Specification No. 1504894 there is described aprocess for the extraction of metal values from aqueous solutions ofmetal salts using an extractant in the presence of halogen or psuedohalogen anions. The halogen or pseudohalogen anion is essential to theextraction process and takes part in the formation of a neutral complexsuch as [L₂ MX₂ ] where L is the extractant, M is a divalent metalcation and X is a halogen or pseudohalogen ion. The process isparticularly applicable to solutions of salts of metals such as copper,cobalt and zinc. Included amongst the extractants disclosed are certainimidazoles.

The solvent extraction processes described in United Kingdom PatentSpecification No. 1504894 are radically different from solventextraction processes used to recovery metals such as uranium. Thusuranium is present in the leach solution in the form of the uranyloxyion, not as a uranium salt, and furthermore the extraction takesplace by the ion exchange of charged ionic species, and not by theformation of an uncharged complex species. Uranium cannot be extractedfrom a solution containing a significant concentration of halide orpseudohalogen ions using nitrogen-based extractants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an extraction isotherm;

FIG. 2 is a strip isotherm for uranyl ions using 1-(2-hexyldecyl)benzimidazole as the imidazole compound.

According to the present invention there is provided a process for theextraction of metal values from an aqueous solution of a metal oxyion inthe absence of halogen or pseudohalogen anions which comprisescontacting the aqueous solution with a solution in a water-immiscibleorganic solvent of an imidazole of formula (I) ##STR1## where R₁ is analiphatic, cycloaliphatic, aryl or aralkyl group, R₂ is a hydrogen atomor a methyl ethyl or vinyl group, and R₃ and R₄, which may be the sameor different, are each a hydrogen, a hydrocarbon group attached directlyor through an oxygen atom to the rest of the molecule, or both R₃ and R₄together with the two carbon atoms joining them form an optionallysubstituted ring, said compound containing a total of at least fivealkyl, alkenyl or cycloalkyl carbon atoms in the groups R₁, R₂, R₃ andR₄ and separating the aqueous phase from the organic phase whichcontains in solution a complex of the metal oxyion and the imidazole.

According to a further aspect of the present invention there is provideda process for the extraction of metal values from an aqueous solution ofa metal oxyion in the absence of halogen or pseudohalogen anions whichcomprises contacting the aqueous solution with a solution in awater-immiscible organic solvent of an imidazole of formula (II) or abenzimidazole of formula (III) ##STR2## where R₁ is an aliphatic,cycloaliphatic, aryl or aralkyl group, R₂ is a hydrogen atom or a methylor ethyl group, and R is hydrogen or an alkyl group or an alkoxy groupor a nitro, chloro, carboxyalkyl or acyl group, the total number ofcarbon atoms in the groups R₁, R₂ and R being at least five, andseparating the aqueous phase from the organic phase which contains insolution a complex of the metal oxyion and the imidazole orbenzimidazole.

For the extraction of uranium R is preferably hydrogen or an alkyl groupor alkoxy group.

It is preferred that the groups R₁, R₂ and R contain a total of at least7, and especially at least 9 carbon atoms, since such groups increasethe solubility of the imidazole or benzimidazole and the metal complexformed therefrom in the water-immiscible solvent. It is especiallypreferred that the more bulky substituents be located in the 1-position(R₁). Thus R₂ is preferably hydrogen, and R is preferably hydrogen or alower alkyl or alkoxy group having from 1 to 4 carbon atoms. It ispreferred to avoid bulky substituents in the 4-position, and thesubstituent R is preferably in the 5-position and/or in the 6-position,or a mixture of two compounds is used in which R is in the 5-positionand 6-position respectively. It may be convenient to have as group R amethyl or methoxy group in the 5-position, since this may facilitatepreparation of the compound. It is not usually advantageous to have morethan a total of 25 carbon atoms in the groups R₁, R₂ and R.

R₁ is preferably an alkyl group having from 7 to 25 carbon atoms.Especially preferred are branched chain alkyl groups. Isomeric mixturesof branched chain alkyl groups are especially suitable in achieving goodsolubility for the extractant and its complex with the metal. Examplesof suitable groups R₁ are branched chain nonyl groups, branched chainisodecyl groups in which the source of the decyl groups is decanolobtained by carbonylation of propylene trimer; branched chain dodecylgroups and branched chain tridecyl and hexadecyl groups.

Especially useful results have been obtained when R₁ is a hexadecylgroup, preferably a branched chain hexadecyl group having the structure##STR3## where R' and R" are alkyl groups containing approximately equalnumbers of carbon atoms, for example the 2-hexyl-decyl group: ##STR4##It is found that imidazoles and benzimidazoles having this substituentin the 1-position show good organic phase solubility of the extractantand its copper complex, low aqueous phase solubility, and good phasedisengagement. The C₈ H₁₇ and C₆ H₁₃ groups of the 2-hexyl-decyl group(IV) may themselves be branched, and may consist of an isomeric mixtureof alkyl groups. Surprisingly, however it is found that excellentresults are also obtained when the C₈ H₁₇ and C₆ H₁₃ groups of thebranched 2-hexyl-decyl group (IV) are each straight chain groups, sothat the 2-hexyl-decyl group is a single species rather than an isomericmixture.

The imidazoles and benzimidazoles may be prepared by any of theconventional and well-known general methods applicable to compounds offormula (I), (II) and (III).

The process of the present invention comprises the steps of (1)contacting the aqueous solution of the metal oxyanion with a solution ina water-immiscible organic solvent of the imidazole or benzimidazole and(2) separating the aqueous phase from the organic phase which containsin solution a complex of the metal oxyanion and the imidazole orbenzimidazole. Preferably the metal-containing organic phase is furthertreated by the steps of (3) contacting the organic phase from step (2)with an aqueous solution of a stripping agent and (4) separating theorganic phase from the aqueous phase containing the metal in the form ofa complex with the stripping agent.

The stripping agent is a species which in the aqueous phase forms acomplex with the metal oxyion which is more stable than the organicphase complex of the metal oxyanion and the imidazole or benzimidazole.Examples of suitable stripping agents are ammonium sulphate solution,sodium chloride and sodium carbonate. Nitric acid may be used but isgenerally less favoured as it tends to have too great an affinity forthe imidazole or benzimidazole which must then be stripped of nitricacid before the (benz)-imidazole can be recycled to extraction.

The stripped aqueous phase solution of the metal from step (4) may betreated in any suitable manner to recover the metal in the desired finalform. For example the aqueous phase stripped uranium solution may betreated with aqueous ammonia to precipitate ammonium diuranate which maybe subsequently heated to convert it to uranium oxide or "yellow cake".

The process of the present invention may use conventional solventextraction plant and techniques. For example the contacting of theaqueous solution of the metal oxyion with the organic phase containingthe imidazole or benzimidazole and the subsequent separation of thephases [steps (1) and (2)--the extraction stage] may take place inconventional mixer-settlers as may the contacting of themetal-containing organic phase with the aqueous stripping solution, andthe subsequent separation of the phases [steps (3) and (4)--thestripping stage]. In general the extraction and stripping will both takeplace in a plurality of individual extraction and strip stages, eachwith its own mixer-settler. The determination of the number ofextraction stages and the number of strip stages desirable with anygiven extractant is a well established procedure. The process of thepresent invention is found to achieve good recovery of metals such asuranium with the use of fewer extraction and/or strip stages than arerequired for conventional processes.

The water-immiscible organic solvent conventionally used for solventextraction is kerosene, but other solvents such as aliphatichydrocarbons and chlorinated hydrocarbons may be used if desired.

Modifiers may be added to the extractant if desired to minimise anyformation of a third phase which would interfere with the separation(phase disengagement) steps (3) and (4). Examples of such modifiersinclude alcohols, for example isodecanol or tridecanol, and alkylphenols such as nonyl phenol. Modifiers such as these may also have thedesirable effect of increasing the solubility of the extractant and themetal complex with the extractant in the organic solvent, or of reducingthe solubility of the extractant in the aqueous phase.

The process of the present invention is especially suitable for therecovery of metals leached from their ores in the form of solubleoxyions. However, the process may be applied to aqueous solutions ofmetal oxyions from any source, including for example waste solutionsfrom chemical processes or solutions derived from waste metals.

Taking uranium as an example, uranium is usually leached from its oresby sulphuric acid. Whilst the scope of the present invention is not tobe taken as being limited by any particular theory, it is believed thaturanium is extracted essentially as the uranyl ion (UO₂ ²⁺) which ispresent in solution in the form of various complexes with sulphate ionsuch as [UO₂ (SO₄)₃ ]⁴⁻. It is also believed that the organic phaseimidazole or benzimidazole (AN) is at least partly protonated by theaqueous phase acid:

    AN.sub.org +H.sub.2 SO.sub.4aq →[(ANH).sub.2.sup.+ SO.sub.4.sup.2- ].sub.org                                                 (1)

The extraction of the uranium aqueous phase species into the organicphase is then thought to take place by an ion exchange reaction such as:

    [UO.sub.2 (SO.sub.4).sub.3 ].sup.4-.sub.aqu +2[(ANH).sub.2.sup.+ SO.sub.4.sup.2- ].sub.org →[(ANH).sub.4.sup.+  (UO.sub.2 (SO.sub.4).sub.3).sup.4- ].sub.org +2SO.sub.4.sup.2-.sub.aqu(2)

It will be seen that the complex of the metal species with the imidazoleor benzimidazole extractant takes the form of an ion pair of salt of theprotonated form of the extractant.

If there is insufficient acid present in the aqueous metal solution tobe treated, the protonation reaction (1) above does not proceedsatisfactorily. Conversely, if there is excessive acid present, abisulphate salt may be formed. It is preferred therefore that theprocess of the present invention be applied to solutions of metaloxyions having an acid concentration in the range 1 to 30 grams perliter sulphuric acid. With uranium, for example, conventional leachingprocesses for the treatment of the ore produce solutions having an acidconcentration typically in the range 3 to 20 grams per liter sulphuricacid.

The process of the present invention may be applied to metals which forman oxyion species in solution. Examples of such metals are tungsten(forming a metal tungstate), molybdenum (forming a molybdate), chromium(forming a chromate) and vanadium (forming the vanadyl ion or avanadate). Some metals may form an oxyion species in one oxidation stateand a simple salt in another oxidation state; the process of the presentinvention applies only to these metals when in the form of an aqueousoxyion species.

EXAMPLE 1

1-isodecylbenzimidazole was prepared by the following method:

Benzimidazole (101.5 parts), potassium hydroxide pellets (48.6 parts)and ethyl alcohol (740 parts) were placed in a reaction flask fittedwith stirrer, dropping funnel and reflux condenser. To the stirredmixture was added slowly 230 parts of isodecyl bromide and the reactantsheated at reflux for 2 hours. The flask was then allowed to cool, thecondenser set for distillation and the alcohol distilled off. To theflask was added 500 parts toluene, 400 parts of 5% aqueous sodiumchloride solution, the contents stirred for 5 minutes and transferred toa separating funnel. The aqueous phase was run off and discarded and theorganic phase washed with successive portions of 5% sodium chloridesolution until neutral. This was followed by two washes with 2 molarhydrochloric acid containing 5% sodium chloride followed by furtherwashed with 5% sodium chloride solution until washes were neutral.

The toluene was stripped off the organic product and the residuedistilled under reduced pressure to give 182.2 parts of product boilingrange 146°-162° C./0.06 mm Hg 82% yield based on benzimidazole.

The ability of the 1-isodecylbenzimidazole to extract uranium wasevaluated as follows:

A solution was prepared containing 32.25 g/l (0.125 molar) of theproduct prepared above together with 20 g/l p-nonyl phenol in Escaid 100(a kerosene type hydrocarbon solvent developed as a carrier in solventextraction processes). Portions of this solution were contacted byvigorous stirring for 15 minutes with equal volume portions of 1.00 g/luranium as uranyl sulphate in aqueous solution at various initial pHvalues, the required initial pH being obtained by the addition ofsulphuric acid.

The organic and aqueous phases were separated and the organic phaseanalysed for uranium. The results are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        pH (initial)                                                                            2.5    2.25    2.0  1.75  1.50 1.25 1.0                             mg/l uranium                                                                            58.9   218     397  818   1069 1388 1175                            in organic                                                                    phase                                                                         ______________________________________                                    

EXAMPLE 2

1-(2-hexyl decyl)benzimidazole was prepared by the following method:

Potassium hydroxide pellets (91.1 parts) were dissolved in methylatedspirits 74° OP (820 parts) in a flask fitted with stirrer, thermometer,reflux condenser and dropping funnel. Benzimidazole (191.9 parts) wasthen added and stirring maintained at 30° C. until it was dissolved.1-(2-hexyl decyl) bromide was added from the dropping funnel withstirring over 15 minutes, the mixture heated to reflux for 6 hours andallowed to cool overnight. The 1-(2-hexyl decyl)bromide was preparedfrom 2-hexyldecyl decanol supplied by the Enjay Chemical Company anddescribed by them as being "a branched chain C₁₆ primary alcohol derivedfrom petroleum. The primary carbinol group is located approximately inthe middle of a long branched chain. It is made up of an array ofisomeric structures". The condenser was then set for distillation, thealcohol distilled off and the flask allowed to cool. To the crudeproducts in the flask were added 1000 parts of cold water and 800 partsof light petroleum (40°-60°) stirring maintained for 15 minutes todissolve the precipitated potassium chloride and the organic and aqueousphases separated.

The organic phase was washed with several portions of M/1 hydrochloricacid in 10% sodium chloride solution followed by 10% sodium chloridesolution then M/1 sodium hydroxide in 110% sodium chloride solution,followed by several washed with 10% sodium chloride solution.

The organic phase was separated off, dried over anhydrous magnesiumsulphate, filtered, and the solvent distilled off to give 546.8 parts ofcrude product. This was distilled under reduced pressure to give 393.5parts of a fraction boiling at 175°-184° C./0.1 mm.

The 1-(2-hexyl decyl)benzimidazole was evaluated as an extractant foruranium by the following method.

A solution was prepared containing 42.75 g/l (0.125M) of the1-(2-hexyldecyl)benzimidazole prepared as described above in Escaid 100.Portions of this solution were equilibrated by stirring for 15 minuteswith equal volume portions of solutions containing 0.972 g/l uranium asuranyl sulphate plus various amounts of sulphuric acid to give a rangeof initial pH values. The phases were then separated and the aqueoussolutions analysed for uranium. The amount of uranium remaining inaqueous solution after one contact with the extractant solution atvarious initial pHs is shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        pH (initial)                                                                           2.70   2.42   2.13 1.84 1.58 1.32 1.04 0.92                          mg/l uranium                                                                           964    960    940  920  505  168  *    *                             remaining in                                                                  aqueous                                                                       phase                                                                         ______________________________________                                         *Down to initial pH 1.32 separations were clean and rapid. Solutions at       initial pH 1.04 and 0.92 showed third phase formation which interferred       with phase separation, and these solutions were not analysed.            

EXAMPLE 3

The procedure of Example 2 was repeated but with the inclusion of 20 g/lof 4-nonyl phenol in the Escaid solution of1-(2-hexyldecyl)benzimidazole. Again, portions of the extractantsolution were contacted with solutions containing 0.972 g/l uranium plusvarious amounts of sulphuric acid, the phases separated and the aqueoussolutions analysed for uranium content. The results are shown in Table3.

                  TABLE 3                                                         ______________________________________                                        pH (initial)                                                                          2.70   2.40   2.13 1.84 1.58 1.32 1.04 0.92                           mg/l ura-                                                                             960    953    935  863  388  4    2    Some                           nium                                           de-                            remaining                                      tected                         in aqueous                                                                    phase                                                                         ______________________________________                                    

No third phase formation was observed. It will be noted that thepresence of the 4-nonylphenol in the organic phase results in higheruranium extraction (lower residual uranium in the aqueous phase) ascompared with Example 2 in which 4-nonylphenol was not added.

EXAMPLE 4

Mixed 5/6-methyl-1-(2-hexyldecyl)benzimidazole was prepared from amixture of 5- and 6-methylbenzimidazole by reaction with1-(2-hexyldecyl)bromide by the method described in Example 2. The1-(2-hexyldecyl)bromide was prepared from 1-(2-hexyl)decanol supplied byEfkay Chemicals, and the C₆ and C₈ groups were both stated to beessentially straight chain groups in the structure: ##STR5##

A 0.125 molar solution was made up (46.7 g/l) incorporating in addition20 g/l 4-nonyl phenol in Escaid 100. Portions of this solution werecontacted with equal volume portions of a uranium solution containing0.972 g/l uranium as uranyl sulphate and various amounts of sulphuricacid. After equilibrating, samples of the aqueous solution were removed,analysed for uranium, and the amount of uranium extracted calculated bydifference. The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        pH (initial)        1.32    1.58    1.84                                      mg/l uranium in organic phase                                                                     968     852     92                                        ______________________________________                                    

EXAMPLE 5

1-(2'-hexyldecyl)-2-ethyl-4,5,6-7-tetrahydro benzimidazole (V) wasprepared as follows: ##STR6##

A mixture of 2-ethylbenzimidazole (20 parts), glacial acetic acid (100parts) and 3% palladium on carbon catalyst (3 parts) is hydrogenated, ina glass liner within a rocking steel autoclave, at 50 Atmospherespressure for 5 hours at a temperature of 180° C. The mixture is cooledand filtered and then diluted with water (400 parts) and made alkalinewith sodium hydroxide, when 2-ethyl-4,5,6,7-tetrahydrobenzimidazole isprecipitated. After collecting the precipitate, washing it with aqueousammonia (density 0.88), and drying under reduced pressure, 15.3 parts,mp 294°-205° C., are obtained.

2-Ethyl-4,5,6,7-tetrahydrobenzimidazole (14.3 parts), 2-methoxyethanol(30 parts), 1-bromo-2-hexyldecane (35 parts) and sodium carbonate (5.7parts) are stirred together and heated at 125°-135° C. below a refluxcondenser for 15 hours. The reaction mixture is cooled and extractedwith a mixture of toluene (150 parts) and ethyl acetate (50 parts) andthe extract is washed successively with water, dilute sulphuric acid,dilute sodium hydroxide, and water, and then distilled yielding1-(2'-hexyldecyl)-2-ethyl-4,5,6,7-tetrahydrobenzimidazole (V) (16.6parts), b.p. 185°-200° at 0.2 mm pressure.

A solution was prepared containing 48.6 g/l (V) (0.125 molar) plus 20g/l 4 nonyl phenol in Escaid 100. Portions were contacted with equalvolume portions of a uranium solution containing 0.972 g/l uranium asuranyl sulphate and various amounts of sulphuric acid. Afterequilibrating, samples of the aqueous phase were removed, analysed foruranium, and the amount of uranium extracted calculated by difference asbefore. The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Initial pH          1.32    1.84    2.42                                      mg/l uranium in organic phase                                                                     969     887     372                                       ______________________________________                                    

EXAMPLE 6

1-(2'-hexyldecyl)-4,5,6,7-tetrahydrobenzimidazole (VI) was prepared asfollows: ##STR7##

Hydrogenation of 1-(2'-hexyldecyl)benzimidazole (26.1 parts) is carriedout as described in Example 5 except that a temperature of 200° C. and areaction time of 8 hours are allowed. The product is a liquid yielding21 parts of 1-(2'-hexyldecyl)-4,5,6,7-tetrahydrobenzimidazole, b.p.180°-120° C. (0.1 mm) on distillation.

An extractant solution was prepared containing 43.7 g/l of (VI) (0.125molar) and 20 g/l 4-nonyl phenol. On contact with 0.972 g/l uranium atvarious pHs as before, the results shown in Table 6 were obtained.

                  TABLE 6                                                         ______________________________________                                        Initial pH                                                                              1.32    1.58    1.84  2.13  2.42  2.70                              mg/l uranium                                                                            969     966     932   544   312   262                               in organic                                                                    phase                                                                         ______________________________________                                    

EXAMPLE 7

A mixture of 1-decyl 2 alkylbenzimidazoles (VII) ##STR8## where R is amixture of C₇, C₈ and C₉ alkyl groups was prepared as follows:

A commercial mixture of straight and branched chain octanoic, nonanoicand decanoic acids ("Novadel 810") is heated at 200° with o-phenylenediamine to give a mixture of 2-alkylbenzimidazoles melting at 145°-149°C. This mixture (48 parts) is stirred and boiled under reflux withethanol (80 parts), potassium hydroxide (14 parts), and isodecyl bromide(46.5 parts, which is prepared from commercial isodecanol, itself amixture of branched primary decanols obtained via trimerisation ofpropane). After 8 hours the mixture is cooled, adjusted to pH 8 withconcentrated hydrochloric acid, treated with charcoal, filtered, andtheir distilled under reduced pressure. The product (40.8 parts) is thefraction distilling at 200°-209° C. at 0.08 mm pressure. By titration ofa sample with perchloric acid in acetic acid it is found to have anaverage molecular weight of 346.

As in previous examples, 0.125 molar solutions (43.3 g/l) of VII wasprepared plus 20 g/l 4-nonyl phenol in Escaid 100. Contacted with 0.972g/l uranium at various pHs as before. The results are shown in Table 7.

                  TABLE 7                                                         ______________________________________                                        Initial pH        1.32     1.58     1.84                                      mg/l uranium extracted                                                                          687      622      62                                        ______________________________________                                    

EXAMPLE 8

A solution was prepared containing 42.75 g/l (0.125 molar) of1-(2-hexyldecyl)benzimidazole, and 20 g/l of 4-nonyl phenol in Escaid100 kerosene type solvent. The preparation of the1-(2-hexyldecyl)benzimidazole was as described in Example 2, except thatthe 2-hexyl decanol starting material, supplied by Efkay Chemical Co.,contained essentially straight chain C₆ and C₈ groups in the structure:##STR9## Portions of this solution were equilibrated by vigorousstirring for at least 15 minutes at various volume ratios with portionsof an aqueous solution containing 0.915 g/l uranium as uranyl sulphateplus 10 g/l sulphuric acid. The phases were then separated, the amountof uranium remaining in the aqueous solutions analysed and the amountextracted in to the organic phase calculated by difference. Thedistribution of uranium at various volume ratios is shown in Table 8.

                  TABLE 8                                                         ______________________________________                                        Ratio aqueous                                                                          1/1    1.5/1  3/1  4/1  5.7/1                                                                              8/1  9/1  18/1                          volume to                                                                     organic phase                                                                 volume                                                                        mg/l uranium                                                                           913    1365   2636 3355 3827 4100 4230 4680                          in organic                                                                    phase                                                                         mg/l uranium                                                                           2      5      36   76   240  365  445  655                           in aqueous                                                                    phase                                                                         ______________________________________                                    

At not stage was any precipitate of 1-(2-hexyl decyl) benzimidazole orof its complex with uranium observed.

To illustrate the ease of recovery of uranium from the loaded extractantsolution (i.e. stripping), the extractant solution described above wasloaded with 5.04 g/l uranium by contact with an acidic uranyl sulphatesolution. Portions of this solution were contacted at various volumeratios with portions of an aqueous solution containing 132 g/l ammoniumsulphate (1.0 molar). After vigorous stirring for at least 15 minutes toestablish equilibrium the organic and aqueous phases were separated andeach analysed for uranium. The results are shown in Table 9.

                  TABLE 9                                                         ______________________________________                                        Ratio aqueous                                                                             3:1     1:1     1:2   1:2.8 1:3.6                                 volume to                                                                     organic volume                                                                mg/l uranium                                                                              2       232     1038  1900  2200                                  in organic                                                                    phase                                                                         mg/l uranium                                                                              1650    4800    8000  9800  10,200                                in aqueous                                                                    phase                                                                         ______________________________________                                    

The data of Table 8 are plotted in FIG. 1 as an extraction isotherm andthe data of Table 9 are plotted in FIG. 2 as a strip isotherm. Shown inFIGS. 1 and 2 are McCabe Thiele constructions which are used to predictthe number of separate stages required in extraction and striprespectively to achieve a satisfactory recovery for uranium (see forexample G. M. Ritcey and A. W. Ashbrook in Handbook of SeparationTechniques for Chemical Engineers; P. A. Schweitzer McGraw Hill, 1979,page 114) for a description of the utilisation of the McCabe-Thieleconstruction to predict the number of extraction and strip stagesrequired). The constructions in FIGS. 1 and 2 indicate that for the feedshown (0.915 g/l uranium, 10 g/l H₂ SO₄) treated with an organicsolution containing 0.125 Molar 1-(2-hexyldecyl) benzimidazole in Escaid100 containing 2% w/v of nonyl phenol, two extraction stages and twostrip stages will give a raffinate containing less than 0.01 g/l ofuranium representing a recovery of better than 99%.

EXAMPLE 9 ##STR10##

A 0.125 molar (43.33 g/l) solution of the product of Example 6 wasprepared incorporating 20 g/l 4-nonylphenol in Escaid 100. Variousvolume ratios of this solution and an aqueous solution containing 0.89g/l uranium as uranyl sulphate plus 10 g/l sulphuric acid wereequilibrated, separated and the amount of uranium extracted determinedin Example 8. The results are shown in Table 10.

                  TABLE 10                                                        ______________________________________                                        Ratio volume                                                                              3/1     4/1     5.7/1 8/1   9/1                                   of aqueous to                                                                 volume of                                                                     organic                                                                       mg/l uranium                                                                              2268    3540    4600  4960  4990                                  in organic                                                                    phase                                                                         mg/l uranium                                                                              4       5       78.5  270   335                                   in aqueous                                                                    phase                                                                         ______________________________________                                    

I claim:
 1. A process for the extraction of a metal selected from thegroup consisting of tungsten, molybdenum, chromium and vanadium metalvalues from an aqueous solution containing an oxyanion of at lest one ofsaid metals, the absence of halogen or pseudohalogen anions, and an acidconcentration of 1 to 30 grams per liter sulphuric acid, which comprisescontacting the aqueous solution with a solution in a water-miscibleorganic solvent of an imidazole selected from the group consisting of1-isodecylbenzimidazole, 1-(2-hexyldecyl)benzimidazole,5-methyl-1-(2-hexyldecyl)benzimidazole,6-methyl-1-(2-hexyldecyl)benzimidazole, 1-decyl-2-alkylbenzimidazolewhere the 2-alkyl is a mixture of C₇, C₈ and C₉ alkyl groups,1-(2'-hexyldecyl)2-ethyl-4,5,6,7-tetrahydrobenzimidazole and1-(2'-hexyldecyl)4,5,6,7-tetrahydrobenzimidazole so as to provide anaqueous phase and an organic phase and separating the aqueous phase fromthe organic phase which contains in solution a complex of the metaloxyanion and the imidazole.
 2. A process as claimed in claim 1 whereinsaid organic phase containing a complex of the metal oxyanion and theimidazole, after separation from the aqueous phase, is contacted with anaqueous solution of a stripping agent and the organic phase is thenseparated from the resulting aqueous phase containing the metal as acomplex with the stripping agent. PG,24
 3. A process as claimed in claim2 wherein the stripping agent is ammonium sulphate, sodium chloride orsodium carbonate.
 4. A process as claimed in claim 1 wherein the organicsolution also contains a modifier which is an alcohol or an alkylphenol.
 5. A process according to claim 1 wherein molybdenum isextracted and the imidazole is 1-(2-hexyldecyl)benzimidazole.